The invention relates to control of thyristors and more particularly to linearizing the output of such thyristors.
Thyristors, which include both silicon-controlled-rectifiers (SCR's) and triacs, are solid state switching devices that display a low resistance, or conducting mode and a high resistance, or non-conducting mode. The mode is controlled by a gate input, and thyristors will not conduct an electrical current until a firing voltage, either a sufficiently high voltage called break over voltage or a pulse voltage, is applied to the gate. The former may be called the break over mode and the latter a pulse mode. SCR's and triacs both display this same resistance characteristic. The only difference between the two is that a triac is a bi-directional device, while an SCR is uni-directional.
Thyristors are often used in AC power circuits to control the amount of power supplied to a device. By controlling the supply of power, an operating aspect of the device, such as the speed of electrical motor or the intensity of a light for example, can be regulated. However, when a thyristor is used to control a sine wave signal the thyristor provides non-linear control over the output power to the device. Thus, a linear change in control input to a thyristor will provide a non-linearly changing power output to the controlled device. This is due to the fact that the amount of power that a wave can produce is proportional to the total area encompassed under the wave and that the area under a sine wave changes non-linearly with time. The non-linearity makes power adjustments both difficult and inaccurate, making it much more desirable to have linear control over a device.
The current means of linearizing a thyristor output signal is a ramp and pedestal circuit, such as that illustrated in the General Electric SCR Manual (5th Ed.), paras. 9.5.1 and 9.5.2. This circuit is connected between the thyristor and a potentiometer, or other similar device, that controls the input voltage to the thyristor. It provides a non-linear gate control signal that substantially compensates for or cancels out the non-linearity of an output signal. The circuit operates by first null-wave rectifing a sine wave power signal (which is customarily 120 volts R.M.S.), and thereafter the rectifier current is divided into two portions. One is reduced in magnitude and fed into an accumulating capacitor. The other is reduced in magnitude by a power resistor and clipped by a shorting zener diode, resulting in a pedestal wave. The pedestal wave is then fed through a controlling transistor into the accumulating capacitor, and it is added to the rectified sine wave. The resulting wave is a modified pedestal wave having a sinusoidally-rising plateau. The magnitude or voltage level of the modified pedestal wave depends upon the magnitude of the pedestal wave that is fed into the summing capacitor. This is regulated by the controlling transistor, which connects to the potentiometer. The accumulated voltage in the capacitor discharges through a unijunction transistor when the capacitor voltage reaches a threshold level. The time at which the threshold voltage is reached depends upon the magnitude of the modified pedestal wave which in turn is controlled by the potentiometer. The output of the unijunction transistor is coupled through a transformer to the gate of a thyristor. The discharging voltage from the capacitor causes the thyristor gate to place the thyristor in a conducting mode and it can thus deliver power to a load.
Although the ramp and pedestal circuit substantially linearizes thyristor output, it has several severe shortcomings. First, it has a high voltage reference that poses the hazard of a severe shock to a user. This is due to the use of a full wave rectifier whose output is not referenced to ground, which in a conventional 120 volt AC circuit would place the controlling potentiometer that is manipulated by a user about 60 volts above ground. One solution that is used by the industry is an isolation transformer following the full wave rectifier. However, it consumes a large amount of power and introduces distortion into the circuit. Second, the power resistor in the pedestal wave portion of the circuit dissipates a large amount of heat and thus wastes considerable power. Third, the rectified sine wave portion of the circuit does not produce a perfect full-wave rectified current due to distortion introduced by the charging of the accumulating capacitor.
It is therefore an object of the invention to provide a thyristor control circuit that overcomes the problems of the prior art.